Method and device for the purification, especially desalination, of water

ABSTRACT

Water processing method, in particular for producing fresh water from salt water by membrane distillation. In comparison with previously known methods, a significant reduction in investment cost and operating cost can be achieved by the combination of the following measures:
         The water to be processed is kept in a supply chamber the wall of which is formed at least in part by a hydrophobic membrane being permeable for water vapor.   A hydrophilic membrane having a greater thickness in comparison with the hydrophobic membrane and a lower thermal conductivity per unit area runs in parallel with the hydrophobic membrane.   By the pumping action a vapor pressure difference is produced between the water to be processed and the fresh water so that the membrane distillation is driven by the vapor pressure difference resulting from the pumping action, the water condensing in the pores of the hydrophilic membrane.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 35 USC § 371 National Phase Entry Application fromPCT/DE2004/000855, filed Apr. 24, 2004.

The present invention relates to a water processing method and acorresponding apparatus. The most important application is for producingfresh water (clear water) from salt water (especially seawater orbrackish water). Hereafter reference is made to this field ofapplication as an example. However, the invention is also suitable forother applications in which the object is to obtain purified water bydistillation from contaminated water. This includes in particularpurification of water contaminated with bacteria or viruses (e.g.,wastewater or river water).

Desalination of seawater and other water processing methods are of greatimportance in supplying drinking water to the world's population. Of allthe surface water on the planet, only approximately 2.5% is fresh water.Of this, approximately 80% is bound as moisture in soil or frozen in thepolar icecaps, so only approximately 0.5% of the total surface water isavailable as drinking water. Furthermore, drinking water supplies arevery unevenly distributed. Therefore, a large portion of the planet'spopulation suffers from water shortage.

To overcome this problem, numerous methods have been proposed fordesalination of seawater. Some of the requirements to be met aredifficult, because seawater has a salt content of approximately 30 g/L,whereas according to the World Health Organization (WHO), the saltcontent of drinking water must not exceed 0.5 g/L. In conventionaldistillation methods, salt water is evaporated by applying heat and thencondensed again on a cooling surface. To reduce the cost of the highenergy consumption, there have been many attempts to use solar energy asthe energy source. A special type of such a system (“solar still”) isdescribed in the publication Bouchekima et al.: “The performance of thecapillary film solar still installed in South Algeria” from Desalination137 (2001) 31-38.

Improved condensation of the evaporated water is achieved with thissolar still by using a capillary structure on the top side of thecondensation surface.

In addition, various membrane methods have been under discussion for along time. These include reverse osmosis in which salt water is forcedunder pressure through a membrane whose pores are of a size such thatthe salt is retained. Another method is electrodialysis in which twoelectrodes are immersed in an electrolyte solution. In the electricfield of a DC-voltage, ion migration occurs in the salt water. Byconnecting cation and anion exchange membranes in an alternating seriesbetween the electrodes of an electrolysis cell, it is possible to directthe ion flow so that there is an increase in the concentration ofelectrolyte in the outer chambers while there is a decrease inconcentration in the central chamber, i.e., a desalination effectoccurs.

The membrane methods also include membrane distillation in which thewater to be processed, which hereafter for the sake of simplicity isreferred to as salt water, is held in a supply chamber whose wall isformed at least in part by a hydrophobic porous membrane. The pore sizeof the membrane, taking into account its hydrophobic properties (i.e.,its surface tension with respect to water) must be such that the saltwater does not fill the pores of the membrane, so the pores of thehydrophobic membrane contain air. In other words, the maximumhydrostatic pressure, at which water will no longer pass through thehydrophobic membrane, also known as the bubble pressure, is lower thanthe pressure occurring in the supply chamber during operation of themethod.

In membrane distillation, the salt water is evaporated through thehydrophobic membrane. The gaseous water condenses on the distillate side(fresh water side). The distillation process (like conventionaldistillation processes) is based on the temperature difference betweenseawater which is heated and condensing fresh water which is cooled.Various membrane distillation methods are described, for example, in:

-   -   U.S. Pat. No. 3,340,186 (P. K. Weyl)    -   U.S. Pat. No. 4,545,862 (Gore et al.)    -   U.S. Pat. Nos. 4,265,713, 4,476,024, 4,419,187 and 4,473,473        (all by Cheng et al.).

With this as a background, the present invention is based on thetechnical problem of providing a method for water processing, inparticular for desalination of water, and a corresponding device whichare characterized by reduced operating and investment cost.

This technical problem is solved by a water processing method in whichthe salt water is held in a supply chamber, the wall of which is formedat least in part by a hydrophobic water vapor-permeable membrane, whichhas a pore size such that the salt water does not fill up the pores andin which a hydrophilic membrane is arranged parallel to the hydrophobicmembrane, the hydrophilic membrane having a greater thickness incomparison with the hydrophobic membrane and having a lower thermalconduction per unit area and in which through a pumping effect adifference in vapor pressure is created between the salt water and thefresh water so that membrane distillation is driven by the vaporpressure difference resulting from pumping action, the water condensingin the pores of the hydrophilic membrane.

The invention also is directed to an apparatus for processing water, inparticular for extracting fresh water from salt water by membranedistillation, the apparatus comprising a supply chamber, the wall ofwhich is formed at least in part by a hydrophobic water vapor-permeablemembrane whose pore size is such that the salt water does not fill upthe pores, a hydrophilic membrane running parallel to the hydrophobicmembrane, the hydrophilic membrane having a greater thickness incomparison with the hydrophobic membrane and having a lower thermalconduction per unit area so that the water condenses in the pores of thehydrophilic membrane near the interface with the hydrophobic membraneand also comprising a pump mechanism by means of which a difference invapor pressure between the salt water and the fresh water is created, sothat the membrane distillation is driven by the vapor pressuredifference resulting from the pumping action, the water condensing inthe pores of the hydrophilic membrane.

The terms “hydrophilic” and “hydrophobic” are to be understood in theconventional sense: A surface is “hydrophilic” when the relativeattraction between the water molecules is less than the attractionbetween the water molecules and the solid surface. In the case of a“hydrophobic” surface, the opposite statement is applicable. The angleof contact between the liquid and a hydrophilic (wettable) surface isless than 90° (in the absence of other forces) but it is greater than90° in the case of a hydrophobic surface.

The hydrophobic membrane should be as thin as possible. Its thickness ispreferably less than 100 μm, but values of at most 10 μm or even at most1 μm are especially preferred. The water vapor permeability usuallyresults from the fact that the hydrophobic membrane is porous with apore size of at least a few nanometers. In the case of an extremely thinhydrophobic membrane, however, the water vapor permeability may also bebased on a permeability of the membrane which cannot be regarded asporosity in this sense. The upper limit of pore size results from theabove mentioned condition regarding the bubble pressure. Taking thislimitation into account, the pore size should be as large as possible.The following synthetic (polymeric) materials are especially suitablefor the hydrophobic membrane: polytetrafluoroethylene, polyvinylchloride.

The hydrophilic membrane need not be homogeneous. In particular, it maycomprise a plurality of layers. Different porous, in particularsynthetic, layer materials are suitable. If the untreated layer materialis hydrophobic, the required hydrophilicity can be achieved by asuitable treatment of the internal surface (i.e., the surface borderingthe pores in the interior of the material). Such methods are known; forexample, hydrophobic polycarbonate is hydrophilized by a thin surfacecoating of polyvinylpyrrolidone.

Especially suitable plastic materials for the hydrophilic membraneinclude the following: cellulose, cellulose acetate, cellulose nitrate,polysulfone and polyether sulfone.

Due to the fact that the hydrophilic membrane is thicker than thehydrophobic membrane (thickness ratio preferably at least 10, especiallypreferably at least 100) and its thermal conduction per unit area ismuch lower than that of the hydrophobic membrane, the temperaturesestablished on both sides of the hydrophobic membrane during operationwill differ only slightly, the temperature on the fresh water side beinga little higher than the temperature on the salt water side.Accordingly, the temperature-related surplus of the vapor pressure onthe fresh water side is only small. Due to a pumping action on eitherthe salt water side or the fresh water side, this surplus of the vaporpressure is overcompensated, resulting in a mass transport through thehydrophobic membrane in the direction of the hydrophilic membrane,whereupon the water condenses in the pores of the hydrophilic membrane.In contrast with the known membrane distillation methods in which a masstransport is achieved by the fact that the fresh water has a lowertemperature than the salt water due to cooling and therefore also has alower vapor pressure, the pressure difference in the inventive methodresults from the pumping action.

Due to the structural measures and process conditions explained here,the pores of the hydrophilic membrane are being filled with fresh waterat the start of distillation and then further condensation takes placein the direct vicinity of the limiting external surface of thehydrophilic membrane facing the hydrophobic membrane. In view of thethermal conditions explained here, the distillation takes placepreferably almost isothermally, the difference between the temperatureof the condensing fresh water and the temperature of the evaporatingsalt water being less than 30° C., preferably less than 10° C.,especially preferably less than 1° C. (“quasi-isothermal membranedistillation”).

Another result of the thermal conditions explained here is that the heatof condensation, which is generated by the condensation on the freshwater side, flows almost completely (with a minimum fraction of 60%)directly through the hydrophobic membrane to the seawater beingevaporated, because the thermal resistance encountered by this heat flowis much lower than the thermal resistance formed by the hydrophilicmembrane which provides thermal insulation. Consequently, a largeportion of the heat of evaporation required for evaporating the seawaterresults from direct recycling of the heat of condensation through thehydrophobic membrane. This greatly reduces the energy consumption of theprocess.

With regard to these factors, the present invention differsfundamentally from some of the United States patents by Cheng citedabove, e.g., U.S. Pat. No. 4,419,242, where the use of a thinhydrophilic layer, which may run on the seawater side and/or on thefresh water side of the hydrophobic membrane and may be either porous ornonporous, is recommended in a different context. Cheng describes howthe salt concentration increases over time in a traditional membranedistillation through a hydrophobic membrane and therefore the wettingproperties of the membrane change. Due to this effect salt water fillsthe pores of the hydrophobic membrane and thereby destroys the requiredvapor barrier (water-logging). This is said to be prevented by the useof the hydrophilic membrane mentioned above. Generally Cheng describes aconventional membrane distillation driven by a sufficiently greattemperature difference between the evaporating salt water and thecondensing fresh water.

In the context of the present invention, the required vapor pressuredifference can be achieved by different pumping methods. In principle itis possible to put the salt water under a sufficiently high pressure byusing a conventional pump. However, this requires that the supplychamber and the membrane have an adequate pressure resistance.

Instead of or in addition to this, a micropump method is preferably usedwithin the scope of the present invention. In this case the pumpingaction is based on forces acting on the water molecules inside the poresof the hydrophilic membrane (“intraporous pump mechanism”). Anelectro-osmotic pump method which is explained in greater detail belowis especially suitable for this purpose.

The present invention is explained in greater detail below on the basisof exemplary embodiments which are depicted in the figures. Theparticulars shown there may be used individually or in differentcombinations to create preferred embodiments of the present invention.

FIG. 1 shows a schematic sectional view of the essential parts of afirst embodiment of a device suitable for implementation of the presentinvention.

FIG. 2 shows a schematic sectional view of the essential parts of asecond embodiment of a device suitable for implementation of the presentinvention.

The water desalination device 1 shown in FIG. 1 consists of a supplychamber 2, a hydrophobic membrane 3, a first electrode 4, a hydrophilicmembrane 5, a second electrode 6 and a fresh water chamber 7. Thisdiagram is highly schematized. In particular, the thickness of themembranes 3, 5 and the electrodes 4, 6 is shown as greatly exaggerated.In reality, these components form a sequence of layers that run paralleland are relatively thin in relation to their (equal) outside dimensions.These layers are attached in such a way that they are in direct contactwith one another.

For securing these layers within the stack of layers labeled as 9 on thewhole, various methods may be used which ensure the required contact anddo not interfere with the passage of vapor and/or liquid which is alsorequired. In particular the entire layer stack 9 may be held together bysuitable mechanical means. As an alternative, sufficiently open-poredlayer-connecting techniques may be used. In particular, the electrodes 4and 6 and the hydrophobic membrane may be applied to the hydrophilicmembrane, which is relatively thick and acts as a carrier, by laminationor some other coating method (e.g., vapor-phase deposition).

The stack of layers 9 is connected to the supply chamber 2 in such a waythat it completely closes an opening of this chamber (not shown in thefigure). It thus forms a part of the walls bordering the supply chamber.Different shapes known in the state of the art would of course bepossible, e.g., prismatic, coiled, tubular forms, etc.

As explained above (and as is customary in membrane distillationmethods), the hydrophobic membrane 3 is made of a material that is notwetted by water so that water vapor can penetrate through its pores 3 a(depicted symbolically in the figure) but liquid water cannot. Watervapor passes through the first electrode 4, which is also porous,whereby it enters the pores 5 a (again depicted symbolically) in thehydrophilic membrane 5. According to the invention, condensation toliquid water takes place in the pores Sa of the hydrophilic membrane 5.This includes cases in which the first electrode 4 is part of thehydrophilic membrane and is designed so that condensation takes place inthe pores of the electrode 4 at the phase boundary with the hydrophobicmembrane. Driven by a pumping action water then passes from thehydrophilic membrane through the second electrode 6 (also porous) andthen enters the fresh water chamber 7 from which it is flows, preferablycontinuously, into a collecting tank (not shown). This may be supportedby a lower pressure generated by pumping.

A first important prerequisite of the inventive function is that thethermal conduction per unit area of the hydrophilic membrane 5 is lower(preferably very much lower) than the thermal conduction per unit areaof the hydrophobic membrane 3. The thermal conduction of a flatstructure such as the membranes used here is proportional to the thermalconductivity of the material used and is inversely proportional to itsthickness. To implement the condition mentioned here, the thermalconductivity of the hydrophobic membrane 3 should be as high as possibleand the thermal conductivity of the hydrophilic membrane 5 should be aslow as possible. However, the choice of these materials is subject tomany other restrictions, e.g., with regard to wetting properties, poresize, porosity and mechanical strength. It is therefore possible that ina concrete case the thermal conductivity of the hydrophobic membrane 3is only insignificantly higher or even lower than the correspondingvalue of the hydrophilic membrane 5.

In the practical implementation of the present invention, the thicknessof the materials used is of great influence. Preferably the layerthickness d of the hydrophobic membrane 3 should be very small.Numerical values were mentioned above. The thickness of the hydrophilicmembrane 5 is preferably at least ten times, especially preferably atleast one hundred times the thickness of the hydrophobic membrane 3. Theabsolute value of the thickness of the hydrophilic membrane 5 ispreferably at least 0.1 mm, especially preferably at least 1 mm. On thewhole, the thermal conduction per unit area through the hydrophobicmembrane 3 should be at least three times the thermal conduction perunit area through the hydrophilic membrane 5.

For the practical success of the present invention, it is also importantthat the hydrophobic membrane 3 has sufficiently high vaporpermeability. It should preferably be at least 100 L/(h·m²) with apressure difference of 60 bar. The pore size of the hydrophobic membrane3 should preferably be at most 100 μm, especially preferably at most 5μm.

As explained previously, another important feature for the function ofthe present invention is that the vapor pressure difference between thesalt water and the fresh water required for membrane distillation ismaintained by pumping action. In the embodiment depicted in the figures,this is accomplished by means of a micropump method which is implementedas an electro-osmotic pump. To this end, a hydrophilic membrane is used,which has a positive or negative surface charge on its internal surface.This condition can be met by a suitable choice of materials. Inparticular, a membrane whose internal surface is coated with an electronexchange material may be considered for use. An electric field isgenerated within the membrane 5 by the two electrodes 4, 6 runningparallel to the limiting external surfaces 10, 11 of the hydrophilicmembrane 5, a suitable voltage being applied to these electrodes duringoperation. In general, a DC-voltage is suitable. However, there are alsomethods in which a pulsating voltage or a modified alternating voltageis used. The electro-osmotic pump action is based on the fact that, dueto the electric charge carriers present on the walls of the pores, adiffuse double layer is formed inside the pores and the ions in theliquid are accelerated in the electric field according to itspolarization and their charge polarity. More information regardingelectro-osmotic pump methods can be obtained from the publications citedbelow in the description of an example. Within the scope of the presentinvention, the first electrode 4 may have either a positive or negativecharge (and the second electrode 6 may then be negative or positiveaccordingly). Which of these polarities is selected in the individualcase will depend on the polarity of the charge on the inside surface ofthe hydrophilic membrane. The electrode polarity must at any rate beselected so that ions are accelerated in the direction away from thehydrophobic membrane, so that water is pumped out of the pores 5 a ofthe hydrophilic membrane towards the fresh water chamber 7.

During operation, the pores 5 a of the hydrophilic membrane 5 are almostcompletely filled so that condensation inside the pores 5 a takes placein immediate proximity to the interface 10 of the hydrophilic membrane 5facing the hydrophobic membrane 3. This is caused by a self-regulatingeffect which can be explained on the basis of the capillary condensationeffect as follows. At the beginning of the distillation process, thewater vapor passing through the hydrophobic membrane 3 condenses in thepores 5 a due to their hydrophilic capillary properties. Thereby thepores 5 a gradually fill up until the meniscus of the condensing freshwater is close to the interface 10. In this state, the efficacy ofdistillation depends on the degree of concave curvature of thecondensation surface (i.e., the meniscus of water in the pores 5 a)which in turn depends on the reduced pressure created by the pumpingaction. Consequently, the concave curvature increases with an increasein reduced pressure due to pumping action so that the efficacy ofcapillary condensation is increased. This causes more water vapor to beresupplied and the curvature of the meniscus tends to become smaller.

This self-regulating behavior is important for the function of thepresent invention because then the location of condensation (theaforementioned meniscus) is very close to the hydrophobic membrane 3(due to its small thickness) and very close to the site of evaporation.Therefore, the quasi-isothermal process mentioned above is achieved,which in turn results in a very low energy consumption.

FIG. 2 shows an embodiment in which the first electrode 4 runs withinthe hydrophilic membrane 5 which consists of two layers 12 and 13. Inprinciple the electrodes 4 and 6 may be localized differently within thedevice, provided the condition is met that the electric field betweenthem acts on the liquid in the pores 5 a of the hydrophilic membrane(when a voltage is applied). For example, the second electrode 6 mayalso be positioned on the side of the fresh water chamber 7 facing awayfrom the hydrophilic membrane 5. The position of the electrodes 4, 6 hasan influence on the self-regulating behavior mentioned above with regardto the meniscus of the fresh water condensing in the pores 5 a. Thearrangement depicted in FIG. 2 may be advantageous in this regard.

As a rule, at least one of the electrodes 4, 6 is fixedly connected tothe hydrophilic membrane 5 and/or integrated therein. Such electrodescan be regarded as components of the hydrophilic membrane 5. Inparticular this means that in the case of an electrode integrated intothe hydrophilic membrane 5 at the interface 10, condensation may takeplace in the part of the membrane 5 formed by the electrode 4.

As mentioned above, the hydrophilic membrane may be made of differentmaterials which usually have approximately spherical pores. However,membrane materials having pores which run essentially in a straight linebetween the interfaces 10 and 11, as depicted in FIGS. 1 and 2, are alsoavailable. Such materials may be used to advantage for the hydrophilicmembrane according to the present invention.

In the embodiment with an electro-osmotic pumping method, as depicted inthe figures, electrochemical effects are to be taken into account. Inparticular, hydrogen may be generated at the cathode if the voltage ishigher than approximately 1.8 V. This can be utilized to producehydrogen in addition to purified water. In this case the electrodefacing the hydrophobic membrane is preferably the anode. It is, however,also possible to reduce or eliminate unwanted electrochemical effects byusing suitable ion exchange membranes which hinder the passage of ionsor gases (e.g., oxygen) that would cause unwanted electrochemicalreactions.

The following example is presented to further illustrate the presentinvention:

To describe the method, various process steps must be taken intoaccount. The sample calculations are based on a mass flow of 30 L/h·m²and a seawater temperature of 17° C.

1. Heat Balance at the Point of Condensation

Water evaporates through the hydrophobic membrane and the subsequentcondensation takes place on the surface of the water-filled pores of thehydrophobic membrane. The associated heat transport can be describedmathematically as follows. The heat loss is assumed to be one part perthousand in this example. The temperature increase is assumed to be 0.14K.

${{Heat}\mspace{14mu}{loss}\text{:}\mspace{14mu}\frac{{QW2} + {QM2}}{QM1}} = \frac{1}{1000}$Temperature  increase:  T2 = T1 + 0, 14KHeat  balance:  Okon = QW2 + QM2 + QM1 = M * Hv where: Q_(kon) = M * Hv${QM1} = {\frac{\lambda_{M1}}{L_{1}}\left( {A_{1} - A_{e1}} \right)\left( {{T2} - {T1}} \right)}$${QM2} = {\frac{\lambda_{M2}}{L_{2}}\left( {A_{2} - A_{e2}} \right)\left( {{T2} - {T3}} \right)}$${QW2} = {{\frac{\lambda_{W}}{L_{e2}}{A_{e2}\left( {{T2} - {T3}} \right)}} + {{Mcp}\left( {{T2} - {T3}} \right)}}$

Qkon=heat of condensation

QM1=thermal conduction through the hydrophobic membrane

QM2=thermal conduction through the hydrophilic membrane

QW1=thermal conduction and transport through the water

λ=thermal conductivity coefficient

A=cross-sectional area

L=thickness of the membrane

M=mass flow

cp=thermal capacity

Hv=heat of vaporization

T1=temperature at the site of evaporation

T2=temperature at the site of condensation

T3=temperature in the bulk of the fresh water

Table 1 shows the results of calculations for a given mass flow of 30L/h·m². The temperature increase in seawater is 1° K. This shows thatthe desired quasi-isothermal process is achieved.

TABLE 1 Seawater in Seawater out Drinking water out T M Conc. T M Conc.T M Conc. [° C.] [kg/s] [g/kg] [° C.] [kg/s] [g/kg] [° C.] [kg/s] [g/kg]17 9.2 × 10⁻³ 36 18 8.3 × 10⁻⁴ 50 17 8.3 × 10⁻³ —

2. Capillary Condensation

The water vapor condenses in the pores of the hydrophilic membrane,forming a curved surface in the capillary. The vapor pressure prevailsabove this surface is pv. This vapor pressure pv is reduced incomparison with the vapor pressure pv0 on a planar surface and is thusthe driving force for condensation of water vapor in the capillary. Thereduced vapor pressure pv can be calculated with the help of the Kelvinequation 2.a and equations 2.b and 2.c. The capillary pressure pk whichmust be applied to draw water out of the capillary is calculated usingequation 2.b and must be at least as great as the reduced pressure whichis needed to reduce the vapor pressure and can be calculated from Kelvinequation 2.a.

Equation  2.a:$p_{K} = {\frac{RT}{V_{L}}\ln\;\frac{p_{V}^{0}}{p_{V}}\left( {{Kelvin}\mspace{14mu}{equation}} \right)}$Equation  2.b: $p_{K} = \frac{2\gamma_{L}\cos\;\delta}{r}$Equation  2.c:$\gamma_{H_{2}O} = {0,1179\;\frac{N}{m}\left( {1 - \frac{T}{T_{{krit}.}}} \right)^{\frac{4}{5}}}$where:

pk=capillary pressure

VL=molar volume of the liquid (water: 18.05 cm³/mol)

pv0=vapor pressure at pk=0

pv=vapor pressure

R=general gas constant 8.31441 J/molK

T=temperature

Tkrit.=critical temperature (water: 647.3 K)

γL=surface tension

δ=wetting angle

r=radius of the capillary

Table 2 shows the values determined for the example calculation. Thevapor pressure pv is reduced by 170 Pa in comparison with the vaporpressure Pv0 of 2300 Pa. With a pore radius of 12 nm, a capillarypressure of 120,000 Pa is achieved. At least this capillary pressuremust be applied by the electro-osmotic micropump.

TABLE 2 Capillary condensation Capillary condensation Pv P_(k) r [Pa][Pa] [m] 2130 120 × 10⁵ 12 × 10⁻⁹

3. Hydrophobic Membrane

The properties of the hydrophobic membrane can be determined with agiven mass flow and the vapor pressure difference Dp resulting from thereduction in vapor pressure due to capillary condensation (equation3.a).

Equation  3.a: $M = {\frac{DMw}{RT}\frac{A\;\Psi}{L\;\tau}\Delta\; P}$where:

M=mass flow

ΔP=pressure difference

R=general gas constant 8.31441 J/molK

T=temperature

D=diffusion coefficient

Mw=molecular weight

Ψ=porosity (Ψ=Ue/U where Ue=empty volume;

U=total volume of the porous membrane)

A=cross-sectional area (Ae=ΨA/√τ, Ae=effective cross-sectional area)

τ=tortuosity factor

L=thickness of the porous membrane (τ=(Le/L)²,

Le: effective length of the pores)

Table 3 shows the thickness of the membrane with a correspondingpressure difference and a preselected porosity and tortuosity. Thisyields a value of 1 μm for the layer thickness.

TABLE 3 Hydrophobic membrane Hydrophobic membrane M A L P [kg/s] [m²][m] [Pa] 8.3 × 10⁻³ 1 10⁻⁶ 170

4. Electro-Osmotic Pump

To retain the driving force for condensation, i.e., the vapor pressuredifference, the condensed water must be transported out of thecapillaries. To do so, the method of the electro-osmotic micropump isused. A mathematical description of this process can be found in C. L.Rice und R. Whitehead (Electrokinetic Flow in a Narrow CylindricalCapillary; J. Phys. Chem. 69 (1965) 4017-4024) and S. Zeng, C. H. Chen,J. C. Mikkelsen Jr. and J. G. Santiago (Fabrication and characterizationof electro-osmotic micro-pumps, Sensors and Actuators B 79 (2001)107-114). The equations yield the maximum volume flow (massflow/density) 4.a, the maximum achievable pump pressure 4.b and themaximum voltage 4.c.

Equation  4.a:${{{{Max}.\mspace{11mu}{volume}}\mspace{14mu}{{flow}\left( {P = 0} \right)}}:\mspace{14mu} V_{\max}} = {\frac{\lambda^{2}i_{EOF}}{ɛϚ}\frac{BF1}{BF2}}$Equation  4.b:${{{Max}.\mspace{11mu}{{pressure}\left( {V = 0} \right)}}\text{:}\mspace{14mu}\Delta\; P_{\max}} = {{- \frac{8\mu\; L\;{\tau\lambda}^{2}i_{EOF}}{{ɛϚ}\; A\;\Psi\; r^{2}}}\frac{BF1}{BF2}}$Equation  4.c:${{{Max}.\mspace{11mu}{{voltage}\left( {V = 0} \right)}}\text{:}\mspace{14mu} U_{\max}} = {\frac{\mu\; L\;{\tau\lambda}^{2}i_{EOF}}{ɛ^{2}Ϛ^{2}A\;\Psi}\frac{1}{BF1}}$

The volume flow V is related to the pressure difference as expressed inequation 4.d.

Equation  4.d:$V = {V_{\max}\left( {1 - \frac{\Delta\; P}{\Delta\; P_{\max}}} \right)}$

The minimum power as a function of pressure and volume flow, not takinginto account irreversible losses (surface conductivity, bulkconductivity) can be calculated according to equation 4.e.

Equation  4.e:${Power} = {{U_{\max}{i_{EOF}\left( {1 - \frac{\Delta\; P}{\Delta\; P_{\max}}} \right)}} = {{\frac{\mu\; L\;\tau}{\lambda^{2}A\;\Psi}\frac{BF2}{{BF1}^{2}}Q^{2}} + {QP}}}$

The following abbreviations have been used in the above equations:

${BF1} = \left( {1 - \frac{2\lambda\;{I_{1}(r)}}{{rI}_{0}\left( {r/\lambda} \right)}} \right)$${BF2} = \left( {1 - \frac{2\lambda\;{I_{1}\left( {r/\lambda} \right)}}{{rI}_{0}\left( {r/\lambda} \right)} - \frac{I_{1}^{2}\left( {r/\lambda} \right)}{I_{0}^{2}\left( {r/\lambda} \right)}} \right)$where In(x)=modified Bessel function of the first kind of the n-thorder:

${I_{n}(x)} = {{i^{- n}{J_{n}({ix})}} = {\sum\limits_{v = 0}^{\infty}{\frac{1}{{v!}{\Gamma\left( {n + v + 1} \right)}}\left( \frac{x}{2} \right)^{{2v} + n}}}}$where:

V=volume flow

ΔP=pressure difference (output-input)

Ψ=porosity (Ψ=Ue/U, Ue=empty volume; U=total volume of the porousmedium)

A=cross-sectional area (Ae=ΨA/√τ, Ae=effective cross-sectional area)

r=effective pore radius of the porous medium

μ=dynamic viscosity

τ=tortuosity factor

L=thickness of the porous medium (τ=(Le/L)²,

Le=effective length of the pores)

ε=permittivity (ε=ε0·εR, ε0=electric field constant, εR: dielectricconstant)

ζ=zeta potential

U=voltage

λ=thickness of diffuse double layer (Debye length)

I1=modified Bessel function of the first kind of the first order

I0=modified Bessel function of the first kind of the zero-th order

κ=conductivity

iEOF=current through EOF

These equations yield the properties of the hydrophilic membrane. Themembrane must have these properties to allow achieving of anelectro-osmotic flow with a suitable pumping force. Values for the poreradius, the thickness and surface charge are obtained in this way.

Table 4 shows the aforementioned membrane properties for the calculatedexample. In addition, it also shows the positive results obtainable bythe method according to this invention. Furthermore, values for themaximum voltage (7 volts) and the resulting current (13A) are alsogiven. The energy consumption amounts to 92 W, which means that only 3Wh is needed per liter of drinking water.

TABLE 4 Electro-osmotic Flow Hydrophilic Membrane A L r ζ Umax Mmax Pmaxi_(EOF) i_(tot) Power [m] [m] [m] [m] [V] [V] [kg/s] [Pa] [A] [A] [W] 110⁻⁴ 6 × 10⁻⁹ 6 × 10⁻⁹ −0.1 7 2.3 × 10⁻¹ 125 × 10⁵ 365 13 92

1. Water processing method, by means of membrane distillation, whereinthe water to be processed is held in a supply chamber, a wall of whichis formed at least in part by a stack of membrane layers which arepermeable to water vapor, said stack comprising a hydrophobic membraneand a hydrophilic membrane, wherein the hydrophilic membrane runsparallel to the hydrophobic membrane and is located on the side of thehydrophobic membrane that is remote from the water to be processed, thehydrophilic membrane has a greater thickness in comparison with thehydrophobic membrane, the hydrophilic membrane has a lower thermalconduction per unit area in comparison with the hydrophobic membrane, avapor pressure difference between the water to be processed andcondensed water is created by a pumping action so that the membranedistillation is driven by the vapor pressure difference resulting fromthe pumping action, the water condensing in the pores of the hydrophilicmembrane, the water pressure difference is at least partially created bythe fact that the condensed water is pumped out of the hydrophilicmembrane by a micropump method including a force acting on the watermolecules in the pores of the hydrophilic membrane, and wherein, inorder to provide an electro-osmotic pump action, two flat electrodes arearranged in parallel with the hydrophilic membranes such that at least alayer of the hydrophilic membrane runs between the two electrodes. 2.Water processing method according to claim 1, characterized in that thedistillation is essentially isothermal, the temperature of thecondensing fresh water being higher than the temperature of theevaporating water to be processed and the temperature difference beingless than 30° C.
 3. Water processing method according to claim 1,wherein the micropump method is an electro-osmotic pump method.
 4. Waterprocessing method according to claim 1, wherein the vapor pressuredifference is at least partially created by the fact that the water tobe processed is pumped with pressure into the supply chamber.
 5. Methodaccording to claim 1, wherein the vapor permeability of the hydrophobicmembrane is at least 100 L/(h m²) at a pressure difference of 60 bar. 6.Method according to claim 1, wherein the thickness of the hydrophobicmembrane is at most 100 μm.
 7. Method according to claim 1, wherein thethickness of the hydrophilic membrane is at least 0.01 mm.
 8. Methodaccording to claim 1, wherein the thickness of the hydrophilic membraneis at least 10 times the thickness of the hydrophobic membrane. 9.Method according to claim 1, wherein the thermal conduction per unitarea through the hydrophobic membrane is at least three times thethermal conduction per unit area through the hydrophilic membrane. 10.Method according to claim 1, wherein the hydrophobic membrane is porousand its pore size is at most 100 μm.
 11. Method according to claim 1,wherein the hydrophilic membrane comprises a plurality of layers and theindividual layers have different properties.
 12. The water processingmethod according to claim 1, wherein the distillation is essentiallyisothermal, the temperature of the condensing fresh water is higher thanthe temperature of the evaporating water to be processed and thetemperature difference is less than 10° C.
 13. The water processingmethod according to claim 1, wherein the distillation is essentiallyisothermal, the temperature of the condensing fresh water is higher thanthe temperature of the evaporating water to be processed and thetemperature difference is less than 1° C.
 14. The method according toclaim 1, wherein the thickness of the hydrophobic membrane is at most 10μm.
 15. The method according to claim 1, wherein the thickness of thehydrophilic membrane is at least 0.1 mm.
 16. The method according toclaim 1, wherein the thickness of the hydrophilic membrane is at least100 times the thickness of the hydrophobic membrane.
 17. The methodaccording to claim 1, wherein the hydrophobic membrane is porous and itspore size is at most 5 μm.
 18. Device for processing water, by means ofmembrane distillation, comprising: a supply chamber, the wall of whichis formed at least in part by a stack of membrane layers which arepermeable to water vapor, said stack comprising a hydrophobic membraneand a hydrophilic membrane located on the side of the hydrophobicmembrane that is remote from the water to be processed, wherein thehydrophilic membrane runs parallel to the hydrophobic membrane, thehydrophilic membrane has a greater thickness in comparison with thehydrophobic membrane and having a lower thermal conduction per unit areaso that the water condenses in the pores of the hydrophobic membrane inproximity to the interface with the hydrophobic membrane; and a pumpdevice capable of creating a pumping action by means of which a vaporpressure difference is generated between the water to be processed andcondensed water so that the membrane distillation is driven by the vaporpressure difference resulting from the pumping action, the watercondensing in the pores of the hydrophilic membrane, wherein the waterpressure difference is at least partially created by the fact that thecondensed water is pumped out of the hydrophilic membrane by a micropumpmethod including a force acting on the water molecules in the pores ofthe hydrophilic membrane, and wherein, in order to provide anelectro-osmotic pump action, two flat electrodes are arranged inparallel with the hydrophilic membranes such that at least a layer ofthe hydrophilic membrane runs between the two electrodes.
 19. Deviceaccording to claim 18, the electrodes optionally being parts of thehydrophilic membrane and the layer running between them having anelectric surface charge.
 20. Device according to claim 18, wherein thesupply chamber is pressure resistant and a pump is provided by which thewater is pumped with pressure into the supply chamber.
 21. A deviceaccording to claim 18, wherein the vapor permeability of the hydrophobicmembrane is at least 100 L/(h m²) at a pressure difference of 60 bar.22. A device according to claim 18, wherein the thickness of thehydrophobic membrane is at most 100 μm.
 23. A device according to claim18, wherein the thickness of the hydrophilic membrane is at least 0.01mm.
 24. A device according to claim 18, wherein the thickness of thehydrophilic membrane is at least 10 times the thickness of thehydrophobic membrane.
 25. A device according to claim 18, wherein thethermal conduction per unit area through the hydrophobic membrane is atleast three times the thermal conduction per unit area through thehydrophilic membrane.
 26. A device according to claim 18, wherein thehydrophobic membrane is porous and its pore size is at most 100 μm. 27.A device according to claim 18, wherein the hydrophilic membranecomprises a plurality of layers and the individual layers have differentproperties.
 28. A device according to claim 18, wherein the thickness ofthe hydrophobic membrane is at most 10 μm.
 29. A device according toclaim 18, wherein the thickness of the hydrophilic membrane is at least0.1 mm.
 30. A device according to claim 18, wherein the thickness of thehydrophilic membrane is at least 100 times the thickness of thehydrophobic membrane.